The 500-year-old reliquary is engraved with the names of the Magi. Photograph: British Museum

A 500-year-old gold reliquary, beautifully engraved with the names of the Magi and images of Christ and St Helena, which was found by a four-year-old playing with his father's metal detector, has gone on display for the first time at the British Museum.

It would once have been brilliantly coloured, with enamel work filling in the letters and decoration, and may once have contained a relic of the cross. It probably dropped from the neck of some wealthy and pious person, and lay undiscovered in the field for half a millennium.

James Hyatt, from Billericay, was four when he found the pendant two years ago one Sunday afternoon in Hockley, Essex, while he was out with his father Jason. He was fortunately probably too young to understand the excited speculation at the time that his discovery could be worth millions, but may get a superior Christmas stocking from the £70,000 paid with grants from the Art Fund charity and the Friends of the museum, shared between the Hyatts and the landowner.

The little locket was jammed shut when found. After conservation work by Marilyn Hockey at the museum, the back panel slid open again for the first time in centuries – but there was nothing inside except some fibres of flax, probably once grown locally.

James's find was genuine buried treasure though. It was officially declared treasure by a coroner's inquest, and has now become one of the permanent treasures of the British Museum's medieval gallery.